The present invention relates to amphiphilic core-shell latexes.
There is an increasing demand for colloidal nanoparticles having an amphiphilic core-shell morphology because of their applications in biotechnology, coatings and adhesive, as well as solid supports. Physical adsorption of hydrophilic biopolymer or synthetic polymers is the dominant approach to prepare such microspheres. However, covalent binding techniques appear to be the most suitable with a view to ensuring irreversible fixation and better orientation of the biomolecules. Furthermore, non-specific adsorption problems for the hydrophobic particle surfaces can be avoided.
In particular, there is an increasing interest in the fabrication of composite micro- and nanoparticles that consist of hydrophobic polymer cores coated with shells of different chemical composition, see F Caruso, R A Caruso, H Mxc3x6hwald, Science, 282, 1111 (1998). In biomedical areas, there is particular interest in polymeric nanoparticles having a hydrophilic biopolymer shell, see F Caruso and H Mxc3x6hwald, J Am Chem Soc, 121, 6039-6046 (1999). Amphiphilic core-shell particles often exhibit substantially different properties than those of the templated core. For instance, they have very different surface chemical composition and hydrophilicity, and can readily be dispersed in water. Applications of such particles are very diverse. They can be used in diagnostic testing, in bioseparations of target proteins via bonding to the particle surface, and as drug reservoirs in controlled release formulations. They can also serve as a support for gene delivery and cell-growth or for a catalyst, and they can be used in coatings and composite materials. Thus, the preparation of nanoparticles having a well-defined amphiphilic core-shell morphology is extremely significant from both a scientific and a technological point of view.
The following five approaches have been used in the preparation of amphiphilic core-shell nanoparticles:
1) Step-wise deposition of polyelectrolytes from dilute solutions onto charged colloidal polystyrene latex particles. For example, multilayer shells have been formed by the alternate adsorption of oppositely charged polyelecrolytes onto positively charged particles, see G B Sukhorukov, E Donath, H Lichtenfeld, E Knippel, M Knippel, H Mxc3x6hwald, Colloids Surfaces A: Physicochem. Eng. Aspects, 137, 253 (1998) and G B Sukhorukov, E Donath, S Davis, H Lichtenfeld, F Caruso, V I Popov, H Mxc3x6hwald Polym. Adv. Tech. 9, 759 (1998)
2) Shell-crosslinked xe2x80x9cknedelxe2x80x9d (SCK) micelles with a core-shell nanostructure have been formed through self-assembly processes of amphiphilic block copolymers, followed by covalent crosslinking of the shells, see K L Wooley, J. Polym. Sci. Part A: Polym. Chern, 38, 1397 (2000). The amphiphilic diblock and triblock copolymers are prepared by either living anionic or living free radical polymerisation methods, see K L Wooley, J Polym. Sci. Part A: Polym. Chem., 38, 1397 (2000) and V Bxc3xctxc3xcn, X S Wang, M V de Paz Bxc3xa1xc3x1ez, K L Robinson, N C Billingham, S P Armes and Z Tuzar, Macromolecules, 33, 1 (2000).
3) Two-stage seeded emulsion copolymerisations. A seed latex is first prepared by emulsion polymerisation of a hydrophobic monomer, followed by the polymerisation of a water-soluble monomer via a seeded swelling batch or a semibatch process, see W Li, H D H Stxc3x6ver, Macromolecules, 33, 4354 (2000), or with reactive seed microspheres, see R Saito, X Ni, A Ichimura and K Ishizu, J. Appl. Polym. Sci., 69, 211 (1998).
4) Using reactive surfactants or macromonomer that are able to copolymerize with monomers. The resulting copolymers typically end up with a thin hydrophilic shell on the particle surface, see S Roy, P Favresse, A Laschewsky, J C de la Cal, J M Asua, Macromolecules, 32, 5967 (1999), and O Soula, A Guyot, N Williams, J Grade, T Blease, J. Polym. Sci. A: Polym. Chem. 37, 4205 (1999], and A Bxc3xacsi, J Forcada, S Gibanel, V Hxc3xa9roguez, M Fontanille, Y Gnanou, Macromolecules, 31, 2087 (1998).
5) Graft copolymerisations of water-soluble monomers onto a functionalised core particle surface. For example, Ce(IV)-initiated grafting of N-(2-methoxyethyl acrylamide) onto poly(styrene-co-2-hydroxyethyl acrylate) particles has been reported, see D Hritcu, W Muller and D E Brooks, Macromolecules, 32, 565 (1999).
In spite of the success of these approaches in the preparation of amphiphilic core-shell nanoparticles, there are still some major drawbacks to these systems. For example:
In the first approach, the deposition procedure is very complicated and time-consuming. After each adsorption step, the free polyelectrolytes need to be removed by repeated centrifugation and washing cycles. In addition, the polyelectrolyes are physically adsorbed on the particle surface via charge interactions. Thus, the shell layer is very sensitive to pH changes.
Tedious multiple step syntheses are required for the preparation of amphiphilic block copolymers, reactive surfactants, macromonomers and the functionalised latex particles used in the second to fourth approaches.
In the third and fifth approaches, the hydrophilic monomers usually have higher reactivity than the matrix monomers, thus resulting in low surface incorporation and formation of a large amount of water-soluble polymers. Furthermore, highly oxidative conditions are required for the grafting processes that prevent the use of biological molecules.
Thus a new technique for making amphiphilic core-shell nanoparticles is extremely desirable from both a scientific and a technological point of view.
The present invention provides amphiphilic core-shell latex nanoparticles. The core is composed of homopolymer of a hydrophobic vinylic monomer, and grafted copolymer of the hydrophobic vinylic monomer. The shell to which the polymer is grafted is hydrophilic, nitrogen-containing polymer.
Thus, we have developed a facile route to prepare a variety of well-defined amphiphilic core-shell latex nanoparticles with covalent linkages. In our approach, a graft copolymerisation of a vinylic monomer onto an nitrogen-containing, water-soluble polymer is conducted in water or other aqueous systems.
In a preferred process, radicals are first generated on the nitrogen atoms of the hydrophilic polymer through interaction with alkyl hydroperoxide or by other means, and then initiate the free-radical polymerisation of vinylic monomer. The hydrophobic side chains of vinylic polymer generated during the reaction phase separate to form latexes of monodisperse core-shell particles with the hydrophobic polymer as the core and the hydrophilic polymer as the shell.
For example, poly(ethyleneimine) (PEI) is a commercially available water-soluble polymer. It contains 25% primary, 50% secondary and 25% tertiary amino groups. It was discovered that the graft copolymerisation of methyl methacrylate (MMA) onto PEI could be readily achieved in water in the presence of a trace amount of an alkyl hydroperoxide (ROOH) at 80xc2x0 C. A nearly quantitative conversion of MMA is obtained in 2 h, giving a stable white emulsion with mean particle sizes ranging from 120 to 135 nm (diameter) and a very narrow size distribution (xcx9c1.1). TEM micrographs clearly reveal that the nanoparticles have core-shell morphology with the PMMA as the core and PEI as the shell. The presence of PEI in the shell layer has been further confirmed with Zeta potential measurements.
The nitrogen-containing hydrophilic polymer can be natural or synthetic. The nitrogen is preferably present as an amine group. Primary amine (xe2x80x94NH2), secondary amine (xe2x80x94NRH), and tertiary amine (xe2x80x94NR2) are the preferred functional groups for this reaction. Structurally, the amino containing polymers may be in the form of linear or cyclic aliphatic or aromatic amine. The amino function may be located in the polymer main chain or in the side chains. Less preferred functional groups are amides including unsubstituted amide (xe2x80x94CONH2), mono-substituted amide (xe2x80x94CONHxe2x80x94R) and disubstituted amide (xe2x80x94CONRRxe2x80x2), which tend to give lower conversion.
In general, biopolymers containing both amino and amide groups and synthetic polymer containing amine groups give high conversion of the monomer and form very stable core-shell nanoparticles with narrow size distribution.
Examples of the nitrogen-containing polymer include synthetic amino polymers such as polyethyleneimine, N-acetyl sugars such as chitosan, or proteins such as casein, gelatin or bovine serum albumin.
The vinylic polymer is prepared using a vinylic monomer. The nature of the monomer is not critical, and for instance it is possible to employ a vinyl monomer, a diene, an acrylate monomer or an acrylamide monomer.
Examples of vinylic monomers include those of formula R1R2Cxe2x95x90CH, where R1 is hydrogen or alky, and where R2 is alkyl, aryl, heteroaryl, halo, cyano, or other suitable hydrophobic group. Preferred groups for R1 include hydrogen and methyl. Preferred groups for R2 include C1-C6 alkyl; phenyl; monocyclic heteroaryl with 4 to 8 ring atoms, more preferably 5 or 6 ring atoms, and with 1, 2 or 3 ring heteratoms, preferably 1 or 2, more preferably 1 ring atom, selected from nitrogen, oxygen or sulfur; chloro; and cyano.
Examples of dienes include those of formula CH2xe2x95x90C(R1)xe2x80x94C(R2)xe2x95x90CH2 where R1 is hydrogen or halogen or alkyl, and where R2 is hydrogen or alkyl, especially C1-C6 alkyl. Preferred groups for R1 include hydrogen, chloride and methyl. Preferred groups for R2 include hydrogen and methyl.
Examples of acrylate monomers include those of formula CH2xe2x95x90CR3COOR4, where R3 is hydrogen or alky, and where R4 is alkyl or substituted allyl, or other suitable hydrophobic group. Preferred groups for R3 include hydrogen and methyl. Preferred groups for R4 include C1-C16, more preferably C1-C12, alkyl which may be straight-chain or branched, and such groups substituted with one or more substituents chosen from unsubstituted amino, monosubstituted amino or disubstituted amino, hydroxy, carboxy, or other usual acrylate substituent. Particular acrylate monomers comprise ethyl (meth)acrylate, isopropyl (meth)acrylate, n-butyl (meth)acrylate, and the like.
Examples of acrylamide monomers include those of formula CH2xe2x95x90CR3COONHR4, where R3 and R4 are as defined.
For forming the core-shell nanoparticles using the vinylic monomer, the nitrogen-containing polymer is preferably dissolved in an aqueous system, either water, acid or alkali or other appropriate system chosen to suit the polymer. The weight ratio of monomer to nitrogen-containing polymer is usually in the range of 1:1 to 10:1, preferably 2:1 to 6:1. In a typical experiment, there is 0.5 to 2 wt/wt % nitrogen-containing polymer, and 2 to 8 or 10 wt/wt % vinylic monomer. A radical initiating catalyst is preferred, notably a hydroperoxide. Hydroperoxide may be used in combination with a metal ion such as ferric ion or with a low molecular weight polyamine. The mole ratio of vinylic monomer to catalyst is preferably more than 1000:1 and suitably around 5000:1. Other catalysts include potassium persulfate and 2,2xe2x80x2-azobis(2-amidinopropane) hydrochloride, Depending on the nature of the hydrophilic polymer, the reaction might proceed at ambient temperature, but usually an elevated temperature is more effective, typically 40 to 95xc2x0 C., preferably 60 to 85xc2x0 C. for a period of say 1 to 4 hours under an appropriate atmosphere such as nitrogen.
Preferred products have a particle size of less than 200 nm, measured as Dn, the number average diameter. Typically the size distribution is narrow, as shown in the accompanying figures. Dn/Dv values as a measure of size distribution are preferably in the range of about 1.1 or 1.2. The polydispersity, Mw/Mn, of the polymerized vinylic monomer is preferably in the range 1.5 to 3, usually around 2.
Thus, in a typical preferred embodiment, the present invention involves a new method to prepare well-defined amphiphilic core-shell nanoparticles via an aqueous graft copolymerization of vinylic monomer onto amine-containing water-soluble polymers including biopolymers and synthetic polymers. In this process, radicals are first generated either through the interaction between nitrogen atom with alkyl hydroperoxide (ROOH) or other catalyst, then initiate the free-radical polymerization of the vinylic monomer. The hydrophobic side chains of vinylic polymer or its homopolymer generated during the reaction phase separate to form latexes of core-shell nanostructure with the hydrophobic component as the core and the hydrophilic polymer as the shell.
This method has several distinct advantages:
1) Simple and convenient method. One step synthesis via alkyl hydroperoxide-induced graft copolymerisation and homopolymerisation of vinylic monomer in water-soluble polymer.
2) High efficiency, only trace amount of alkyl hydroperoxide is required to induce the graft copolymerization. Thus the covalent bonding of the grafted copolymer is produced with only one to three grafting points. This is particularly important for the biomolecule because in this way, most of active sites remain free and unchanged. Furthermore, this approach overcomes the oxidative degradation and high toxicity problems present in the current grafting methods.
3) Covalent linkage of hydrophilic polymer on the particle surface
4) Very versatile, a much wider range of novel biomaterials and synthetic polymers of core-shell particles can be easily prepared.
5) Discrete core-shell nanoparticles with various biopolymer or hydrophilic polymer on the surface can be readily produced with different sizes, compositions, structures, and functions.
6) A core-shell morphology is obtained where the core diameter and shell thickness can be easily altered.
7) No surfactants are required.
8) The use of aqueous-based chemistry.